Every year, stroke and neurotrauma afflict approximately 660,000 and 350,000 North American individuals, respectively, and about 175,000 stroke and 52,000 trauma victims will die (Stroke-American Stroke Association Web Site, 2000; Centers for Disease Control and Prevention, Traumatic injury in the United States: An interim report to Congress, Centers for Disease Control and Prevention, 2001). 4.5 million people live with Alzheimer's disease, with the prevalence expected to triple by 2050 (Hebert et al. (2003) Arch. Neurol. 60:1119-1122). Epilepsy is the third most common neurological disorder after stroke and Alzheimer's disease. It affects 2.3 million Americans of all ages. Approximately 181,000 new cases of seizures and epilepsy occur each year. One in every 10 Americans will experience a seizure at some point in their lives. Three percent will eventually develop epilepsy (Epilepsy Foundation of America Web Site. Thus stroke, CNS trauma, neurodegenerative illnesses and epilepsy are each disorders of major public health significance. Moreover, cardiovascular disorders, pulmonary diseases, and endocrine illnesses such as diabetes count as among the most common causes of non-neurological morbidity and mortality worldwide. Common to all of these common illnesses is damage to cells in target tissues in the nervous, cardiovascular or endocrine system.
The public health consequences of these disorders are significant. For example, in 1998, $3.4 billion was paid in 1999 to just those Medicare beneficiaries that were discharged from short-stay hospitals, not including the long term care for >1,000,000 people that reportedly have functional limitations or difficulty with activities of daily living resulting from stroke (Heart and Stroke Statistics-2004 Update, American Heart Association, 2004). At this time, no therapeutics are available to reduce brain damage resulting from stroke, and this major disorder can be used as an example for the basis of the current invention, though the field of the invention obviously applies to other disorders involving mammalian cell injury.
Stroke is characterized by neuronal cell death in areas of ischemia, brain hemorrhage or trauma. Many lines of evidence have demonstrated that this cell death is triggered by glutamate over-excitation of neurons, leading to increased intracellular Ca2+ and increased nitric oxide due to an increase in nNOS (neuronal nitric oxide synthase) activity. Excitotoxicity is the process by which L-glutamate, the major excitatory neurotransmitter in the mammalian CNS, damages neurons (Olney (1969) Science 164:719-721; Olney and Sharpe (1969) Science 166:386-388). It is established as a predominant neurotoxic mechanism in acute neurological disorders such as stroke, epilepsy and traumatic nervous system injuries (reviewed in Rothman and Olney (1987) TINS 10:299-302; Choi et al. (1988) Neuron 8:623-634; Coyle and Puttfarcken (1993) Science 262:689-695; Lipton and Rosenberg (1994) N. Engl. J. Med. 330:613-622; Hardingham and Bading (2003) Trends. Neurosci. 26:81-89). In brain ischemia, excitotoxic activation of postsynaptic glutamate receptors triggers downstream pathways implicated in subsequent neuronal death (reviewed in Lipton (1999) Physiol. Rev. 79:1431-1568). Of these, Ca2+ influx through N-methyl-D-aspartate (NMDA) glutamate receptors was the process consistently revealed as a key event. In these studies, blocking NMDA receptors permitted neurons destined to die from anoxia to survive (Rothman (1983) Science 220:536-537; Goldberg et al. (1987) J. Pharmacol. Exp. Ther. 243:784-791), and animal research suggested that ischemic brain damage could be treated by this approach (Simon et al., (1984) Science 226:850-852). However, blocking NMDA receptors may be detrimental to animals and humans (Fix et al. (1993) Exp. Neurol. 123:204-215; Davis et al. (2000) Stroke 31:347-354; Ikonomidou et al. (2000) Proc. Natl. Acad. Sci. U.S.A. 97:12885-12890). Moreover, though blocking excitotoxicity was effective in laboratory models of disease, clinical trials of anti-excitotoxic therapies (AET) have generally failed to benefit patients (Davis et al., (1997) Lancet 349:32, Davis et al. (2000) Stroke 31:347:354; Morris et al. (1999) J. Neurosurg 91:737-743; Lees et al. (2000) Lancet 355:1949-1954). The reason for this, in the face of a clear role for excitotoxicity in acute neurological disorders, has remained a mystery (Birmingham (2002) Nat. Med. 8:5; Ikonomidou and Turski (2002) Lancet Neurol. 1:383-386).
The present invention relates to our discovery that processes other than excitotoxicity are responsible for neuronal damage in conditions such as stroke. In neurons exposed to oxygen glucose deprivation (OGD), AET unmasks a lethal cation current IOGD mediated by TRPM7, a member of the transient receptor potential (TRP) cation channel superfamily (Nadler et al. (2001) Nature 411:590-595). In OGD, IOGD is activated by reactive oxygen/nitrogen species (ROS), permitting Ca2+ uptake that further stimulates ROS and IOGD activation. Blocking IOGD or suppressing TRPM7 expression prevents anoxic neuronal death even in the absence of AET, indicating that TRPM7 is an essential mediator of anoxic death. This work defines a new paradigm for understanding anoxic neuronal damage in which excitotoxicity is a subset of a larger framework of mammalian cell injury that involves TRP cation channels.